Joe Pye Weed

Joe pye weed (Eupatorium purpureum)

Description
In this photo you can see the individual flowers more clearly. One of the main distinctions between Eupatorium purpureum (the plant pictured)and E. fistulosum compared to E. maculatum is the number of florets (individual flowers) per involucre (flowerhead). In E. purpureum and E. fistulosum there are approximately 4-7 florets while E. maculatum has about 10-16. This is not an obscure reference, if you unroll the involucre in you hand, it is pretty easy to count the florets.
For more differences, see other Eupatorium photos.